The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing CO and HC emissions is an oxidation catalyst device (“OC”). The OC device includes a flow-through substrate and a catalyst compound applied to the substrate. One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device that may be positioned downstream of the OC device. The SCR device includes a substrate, having a SCR catalyst compound applied to the substrate. A reductant is typically sprayed into hot exhaust gases upstream of the SCR device. The reductant may be a urea solution that decomposes to ammonia (“NH3”) in the hot exhaust gases and is adsorbed by the SCR device. The ammonia then reduces the NOx to nitrogen in the presence of the SCR catalyst. However, the SCR device also needs to reach a threshold or light-off temperature to effectively reduce NOx. Following a cold start of the engine, the SCR device has not attained the respective light-off temperature, and therefore generally may not effectively remove NOx from the exhaust gases. Accordingly, it is desirable to provide an efficient approach for effectively removing NOx from the exhaust gases prior to the SCR device reaching the respective light-off temperature.